Straw-bales can be made from a range of plant fibres not only grass-family species like wheat, rye, barley, blue-grass, and rice, but also flax, and hemp. Bales of recycled materials like paper, pasteboard, waxed cardboard, crushed plastics, whole tires, and used carpeting have also been used or are currently being explored for building.
Basic straw-bales are produced on farms and referred to as “field-bales”. These come in a range of sizes from small “two-string” bales (W460mm x H350mm – H400mm x L800mm – L1200mm) to three-string “commercial bales” (W540mm x H410mm x L1200mm). These sizes range in weight from 18kg to 45kg.
Larger “bulk” bales are now becoming common, H1000mm x W1000mm – W1200mm x L2000mm and even W1200mm x H1200mm x L2400mm, weighing up to a ton. All of these “economy-size” units also offer unique potential for imaginative designers.
A newer trend is the use of high-density recompressed bales, sometimes called strawblocks, offering far higher compression strength. These bales “remade” from field bales in massive stationary presses, producing up to 4 MN of force, were originally developed for cargo-container transport to over-seas markets.
Innovators soon discovered that, where a wall of “conventional field bales” is able to support a roof load of 900 kg/m, the high-density bales can support up to 4500kg/m to 7000 kg/m. This makes them particularly suited to load-bearing multi-storey or “living-roofed” designs.
They are available in a range of sizes from different companies’ presses but L600mm x H600mm x W450mm might be considered “typical”; because they are bound with horizontally ties or straps, at 10mm or 12mm intervals vertically, they may be recut with a chain-saw at a range of heights. They are usually used in “stacked bond”, with the straws running vertically for greatest strength and tied with “re-mesh” on both sides before stucco application.